The present invention relates generally to springs and flexure seals and, more specifically, to a spring design that may include an integral dynamic gas seal.
The present invention falls generally into the fields of springs and seals. However, one of the preferred applications of the present invention is in a thermoacoustic engine or refrigerator. One such thermoacoustic device is shown in FIG. 1. This thermoacoustic device 20 is the subject of U.S. provisional patent application Serial No. 60/372,008, filed Apr. 10, 2002, and a co-pending patent application entitled xe2x80x9cCompliant Enclosure for Thermoacoustic Devices,xe2x80x9d filed Apr. 9, 2003, the entire contents of both of which are incorporated herein by reference. Another thermoacoustic device is shown in U.S. provisional patent application Serial No. 60/371,967, filed Apr. 10, 2002, and a co-pending patent application entitled xe2x80x9cThermoacoustic Device,xe2x80x9d filed Apr. 9, 2003, the entire contents of both of which are incorporated herein by reference.
The thermoacoustic refrigerator 20 includes an outer pressure vessel 50 containing substantially all of the components of the refrigerator, including a compliant enclosure. The compliant enclosure, in turn, houses the thermal components or thermal core 40. These thermal components 40 include a cold heat exchanger 14, a regenerator 16, and a hot heat exchanger 18. These components 40 are supported by a thermally insulating plate 22 that accommodates the passage of heat exchanged fluids through tubes 24, which communicate with thermal sinks or loads outside the pressure vessel 50. A piston 26 is spaced below the thermal components 14-18, with a bellows 28 extending between and interconnecting the piston 26 and the thermally insulated plate 22. A linear motor or actuator 30 is interconnected with the piston 26 by a moving portion 32 of the motor 30. Therefore, the motor 30 is operable to move the piston. In the illustrated embodiment, the moving portion 32 of the motor 30 is interconnected with the piston 26 by a rigid tube 36. A cylindrical spring 34 may be provided between the piston 26 and some stationary portion of the device, such as the insulating plate 22, to adjust the mechanical resonance frequency of the system.
Most of the thermoacoustic engines and refrigerators that that have been constructed and tested to date, and that either use or produce electricity, operate at xe2x80x9cacousticxe2x80x9d frequencies, usually in the range from about 40 Hz to 500 Hz. To operate at these frequencies, the electro-mechanical transduction device (e.g., loudspeaker or linear motor/alternator) requires some supplemental stiffness, such as spring 34, to allow the moving mass of the electro-mechanical device to resonate at the desired operating frequency. Operation at the resonance frequency has been shown to produce optimally efficient electro-mechanical energy conversion [R. S. Wakeland, J. Acoust. Soc. Am. 107(2), 827-832 (2000)].
In some devices, the gas trapped behind the piston, which produces the sound wave in a thermoacoustic refrigerator, provides this supplemental stiffness. The linear harmonic motion of the piston may also be driven by gas pressure oscillations produced by a thermoacoustic engine. The piston is then used to produce electricity when joined to a linear alternator. One such embodiment of a linear alternator is described in U.S. Pat. No. 5,389,844. In other devices, a mechanical spring, usually made of steel, is used to prove the required supplemental stiffness, as taught in U.S. Pat. No. 5,647,216 (linear) and U.S. Pat. No. 5,953,921 (torsional).
Some mechanism must be provided to prevent the passage of gas around the piston that couples the mechanical energy of the electromechanical device to or from the acoustical energy of the thermoacoustic device. Traditionally, either a clearance or flexure seal has been employed. Thermoacoustic device 20 in FIG. 1 makes use of a flexure seal, which takes the form a bellows 28.
A clearance seal is produced by a very narrow gap between the circumference of the piston and the cylindrical bore within which the piston must slide. The clearance seal requires very close tolerance (hence expensive) machining of both the piston and the bore. This clearance seal arrangement is known to produce extraneous energy dissipation when some amount of the gas passes through the gap (know as xe2x80x9cblow byxe2x80x9d loss). The clearance seal can also produce dissipation due to fluid friction produced by fluid (gas) shear caused by the relative motion of the piston and the bore. In addition to the excess dissipation, an asymmetry between the compression and expansion tends to move gas preferentially behind or in front of the piston. This dynamically-induced static pressure difference tends to un-center the piston (known as xe2x80x9cpiston walkingxe2x80x9d). Some additional mechanism must be provided to relieve the accumulating differential pressure, such as a relief valve, or an acoustic bypass.
For electrically-driven thermoacoustic refrigerators, the flexure seal employed most successfully to date has been the metal bellows. The metal bellows has advantages over the clearance seal since it does not permit blow-by and the energy dissipated by mechanical losses produced by the metal""s flexure is negligibly small. The bellows seal is limited by the fact that the bellows material must be quite thin, typically less than one one-hundredth of an inch (250 micrometers) thick, if it is to be capable of an infinite number of excursions that are on the order of xc2x110% of its convolved length [for further details, see Standards of the Expansion Joint Manufacturer""s Association, Inc., 25 North Broadway, Tarrytown, N.Y. 10591]. Since the walls of the bellows must be thin (to reduce deflection stresses), the bellows does not provide the significant elastic stiffness necessary to resonant the moving mass of the electro-mechanical transducer, nor is it always capable of sustaining the large pressure differentials (that produce pressure stresses in the bellows) that would allow it to produce the stiffness required for it to function as a gas spring.
One purpose of the present invention is to provide a cylindrical spring design that may be more compatible with a new xe2x80x9cbellows bouncexe2x80x9d resonant cavity than the flat spring designs taught in U.S. Pat. No. 6,307,287. Such a xe2x80x9cbellows bouncexe2x80x9d resonance cavity is well-suited for compact thermoacoustic engines and refrigerators that utilize electro-mechanical transducers such as moving-magnet motor/alternators. The geometry and function of a xe2x80x9cbellows bouncexe2x80x9d resonant cavity is described in U.S. provisional patent application Serial No. 60/372,008, filed Apr. 10, 2002, and a co-pending patent application entitled xe2x80x9cCompliant Enclosure for Thermoacoustic Devices,xe2x80x9d filed Apr. 9, 2003.
A cylindrical spring design according to the present invention can also act as a dynamic gas seal if the gaps between the elastic xe2x80x9cspring beamsxe2x80x9d are closed with a second compliant material, such as an elastomer (e.g., rubber or silicone adhesive), to prevent the passage of the gas that is being compressed and expanded by the motion of the spring. By sealing the gaps with a low-loss compliant material [e.g., Type I rubber as described by J. C. Snowdon, Vibration and Shock in Damped Mechanical Systems (J. Wiley and Sons, 1968). Chapter 1], the spring can also replace the bellows that has traditionally been used to provide the flexure seal necessary for vibroacoustic compression and expansion the working fluid (e.g., air, inert gas, or mixture of inert gases) in previous thermoacoustic refrigerators as taught in U.S. Pat. No. 5,647,216.
If the low-loss compliant material used for the dynamic gas seal described above is replaced with a high-loss compliant material (e.g., Type II rubber), then a spring according to the present invention is suitable for use as a vibration isolation mount to decouple machinery vibration from transmission though the structure by which the machine is supported.
One embodiment of a spring with an integral dynamic gas seal according to the present invention includes a spring with a pair of spaced-apart ends and a spring body extending therebetween. The spring body includes a perimeter wall formed of a first material and extending in a longitudinal direction. The wall has a plurality of openings defined therein. The openings are provided in a plurality of transverse tiers. Each tier includes a plurality of separate openings each extending along a portion of the perimeter. The openings in each tier are spaced apart so as to define a post between each adjacent opening such that each tier includes a plurality of posts. The openings in each tier are staggered with respect to the openings in adjacent tiers such that each of the openings in each tier overlie one of the posts in an adjacent tier. A closure member closes each of the openings such that the spring body is sealed. The closure member is formed of a second material having a different degree of flexibility than the first material.
A thermoacoustic device according to the present invention includes a housing and a thermal core disposed in the housing. A thermal core includes at least a first and a second heat exchanger. A piston is spaced from the thermal core, with the piston being operable to oscillate with respect to the thermal core. The thermoacoustic device also includes a spring with an integral dynamic gas seal, as discussed above.